EFFECTS OF CO2 ON PYROLYSIS OF WALNUT SHELLS AT HIGH TEMPERATURE AND HEATING RATE IN A HSR

Previous studies have addressed the effects of carbon dioxide (CO2) rich atmospheres, as those established in oxy-firing of solid fuels, on the properties of chars produced from pyrolysis of biomass. The present work complements previous investigation with experiments carried out under high heating rate/high temperature conditions in a purposely designed heated strip reactor (HSR). Walnut shell samples have been heat treated in the HSR in N2 and CO2 at temperatures of 1573-2073 K with holding times of 3s. Primary tar has been analyzed by GC-MS, char has been analyzed by thermogravimetric analysis and Raman spectroscopy. The effect of substitution of N2 with CO2 during heat up of biomass is not straightforward, as it can result in either an enhancement or reduction of char reactivity and structural order depending on the temperature. At 1573 K, CO2 chars are less reactive and more graphitized than N2 chars. In contrast, when the temperature is increased to 2073 K, CO2 chars turn out to be more porous, less graphitized and more reactive than N2 chars. The behavior is discussed within the frame of the competition of pyrolysis, gasification and thermal annealing.